1. Field of the Invention
The present invention relates to a method for applying brazing material to subregions of a metal structure, particularly a metal honeycomb body. A special application is applying brazing material to the inner surfaces of a jacket tube for a metal honeycomb body, which is used, for instance, as a carrier body for catalytically active material for use in cleaning the exhaust gas of motor vehicles. Other applications with comparably fine sheet-metal structures, such as in heat exchangers, are also possible.
Published European Application No. 0 049 489 B has already disclosed a method for applying brazing material to metal structures, in which a pressure-sensitive adhesive is applied and then is acted upon by brazing powder. That pressure-sensitive adhesive remains adhesive even after drying and is meant to evaporate without residue at the brazing temperature. With the aid of such an adhesive, a thin, practically monoparticulate film can be applied, so that the quantity of brazing material to be applied can be metered quite accurately through the particle size of the brazing material particles being used. That reference makes no further statements about the behavior of the pressure-sensitive adhesive at an elevated temperature, which can occur during heat treatment of the structure.
Various brazing material application methods with their various advantages and disadvantages are known from German Published, Non-Prosecuted Application DE 38 18 512 A1, corresponding to Published International Application WO 89/11938, and corresponding to co-pending U.S. applications Ser. No. 07/621,068, filed Nov. 30, 1990 and Ser. No. 08/054,161, filed Apr. 26, 1993. That reference contains a survey of the brazing methods known at that time for honeycomb bodies and similar structures. Once again, however, nothing is said about the properties of the adhesives or binders being used, at elevated temperature.
It is also known from U.S. Pat. No. 4,795,615, in a metal honeycomb body that has a jacket tube, to perform the binding of the actual honeycomb structure to the jacket tube in a striplike region extending approximately circumferentially, for example by placing a brazing material foil in it. However, such a placement of a brazing material foil is not suitable for all applications, since when a honeycomb structure is thrust into a jacket tube, for instance, a brazing material foil is shifted or displaced.
With more-complex components, such as catalyst carrier bodies or heat exchangers, several production steps are often necessary before the desired body has been completely assembled with brazing material applied to it. The problems that arise will be discussed herein by taking a metal honeycomb body with a jacket tube as an example:
In order to enable the production and processing the fine structures of the sheet metal layers being used for a honeycomb body, it is advantageous for the sheets to be coated with a rolling oil, which may include mineral substances, grease substances and process materials. However, before brazing material is applied to the structure produced from the sheet metal layers, such a rolling oil must be at least partially removed, which can be done by a heat treatment. It is especially practical, though, if such a thermal treatment can already be done on the honeycomb body, which is in its completely manufactured form and has its jacket tube. In other words, the sheet-metal structures provided with an oil film must first be inserted into a suitable jacket tube. Since the regions between the jacket tube and the honeycomb body structure that are to be brazed can no longer be provided with brazing material later, the desired subregions in the interior of the jacket tube must already have brazing material applied to them by that time. If that is to be done in a manner known per se by means of an adhesive material and brazing powder, then the brazing powder must continue to stick in the desired regions even during and after a heat treatment. Similar problems also arise with other honeycomb structures, in which certain parts must already have brazing material applied to them prior to the assembly, while other parts must only have that done later.